Fuel cells that are not properly shut down or started up can suffer considerable electrode degradation due to carbon corrosion and loss of catalyst surface area. One simple and inexpensive way to shut down a fuel cell is to purge out fuel gas in the anode flow field. When air is used to purge the fuel gas in the anode flow field, there are air rich region and hydrogen rich region in the anode flow field during such change-over until fuel gas is completely displaced. As a result, a local electrochemical cell is created in the anode section involving hydrogen oxidation in the hydrogen rich region and oxygen reduction in the oxygen rich region. Such local cell formation causes proton ion migration in the electrolyte layer between the two regions of the anode by an indirect path mediated by the cathode. The local cell on the anode thus triggers formation of another local cell across the corresponding regions on the cathode, involving oxygen reduction and carbon oxidation. Such local cell formation and its corresponding electrochemical reactions are illustrated in FIG. 1. Local cell formation causes the potential of the cathode to shift up by as much as 0.6-0.8 volt to as high as 1.6-1.8 V (RHE). It is believed that the oxygen reduction reaction on the anode and carbon oxidation reaction on the cathode are responsible for catalyst surface area loss and carbon corrosion. Loss of catalyst activity and support material corrosion are two important forms of electrode degradation in fuel cell during startup and shutdown. A fuel cell capable of otherwise operating continuously for long period of time can lose significant amount of its power after limited number of startup/shutdown cycles due to electrode degradation.
The use of an inert gas such as nitrogen and the use of an auxiliary load to reduce the cathode potential shift are disclosed in U.S. Pat. Nos. 5,013,617 and 5,045,414. Since fuel cells are subject to numerous start-up/shut down cycles, use of an inert gas is not practical or economical due to additional inert gas supply and connection requirements. Use of air purge during shut-down is disclosed in US Patent Application Publication US 2003/0134164. A hydrogen fuel purge method during start-up was disclosed in US patent Application Publication US 2003/0134165. The use of an anode exhaust recycle loop was disclosed in U.S. Pat. No. 6,514,635. None of the above processes offer instantaneous and complete fuel gas replacement over the entire electrode. The co-existence of hydrogen-rich and oxygen-rich regions is still created in the startup and shutdown cycles. Local cell formation and electrode degradation still occur.
Modifications of catalysts on anode and cathode are disclosed to mitigate the electrode degradation problems during startup and shutdown in U.S. Pat. No. 6,855,453. Such modifications can reduce, but not completely prevent, the degradation caused by local cells. Catalyst modification, however, usually adds cost and may adversely affect the fuel cell operation efficiency.
Additionally, oxygen cross-over from cathode to anode can cause hydrogen starvation and local cell formation, resulting in significant electrode degradation in a similar manner. There is a need to dramatically reduce fuel cell electrode degradation using an inexpensive and practical approach.